For the purpose of environmental load reduction and environmental protection recently needed on a global scale, the cleaning of exhaust gases for reducing the emission of air-polluting materials, and the improvement of fuel efficiency (low fuel consumption) for suppressing the emission of CO2, a cause of global warming, are strongly required in automobiles. To clean exhaust gases, and to improve fuel efficiency in automobiles, various technologies such as the development of engines with high performance and fuel efficiency, the cleaning of exhaust gases, the weight reduction of car bodies, the air resistance reduction of car bodies, efficient power transmission from engines to driven systems with low loss, etc. have been developed and employed.
Technologies for providing engines with high performance and improving their fuel efficiency include the direct injection of fuel, increase in fuel injection pressure, increase in compression ratios, decrease in displacements by turbochargers, the reduction of engine weights and sizes (downsizing), etc., are used not only in luxury cars but also in popular cars. As a result, fuel combustion tends to occur at higher temperatures and pressure, resulting in higher-temperature exhaust gases discharged from engines to exhaust members. For example, the temperatures of exhaust gases are near 1000° C. even in popular cars, like luxury sport cars, so that the surface temperatures of exhaust members may reach 900° C. Thus, exhaust members exposed to higher-temperature exhaust gases are required to have higher heat resistance characteristics such as oxidation resistance, high-temperature strength, thermal deformation resistance, thermal cracking resistance, etc. than before.
Exhaust members with complicated shapes, such as exhaust manifolds, turbine housings, etc. used for gasoline engines and diesel engines of automobiles have conventionally been formed by castings with high freedom of shape. In addition, because of their severe, high-temperature use conditions, heat-resistant, cast irons such as high-Si, spheroidal graphite cast irons and Ni-Resist cast iron (Ni—Cr-containing, cast austenitic iron), heat-resistant, cast ferritic steels, heat-resistant, cast austenitic steels, etc. are used.
Though high-Si, spheroidal graphite cast ferritic irons exhibit relatively good heat resistance characteristics at temperatures up to near 800° C., they are poor in durability at higher temperatures than 800° C. Heat-resistant, cast irons such as Ni-Resist cast iron containing large amounts of rare metals such as Ni, Cr, Co, etc., and heat-resistant, cast austenitic steels have satisfactory oxidation resistance at 800° C. or higher and thermal cracking resistance. However, the Ni-Resist cast iron is expensive because of a large Ni content, and has poor thermal cracking resistance because it has a large coefficient of linear expansion due to an austenitic matrix structure, and because its microstructure contains graphite acting as breakage-starting points. The heat-resistant, cast austenitic steels have insufficient thermal cracking resistance at about 900° C. because of a large coefficient of linear expansion, though not containing graphite acting as breakage-starting points. In addition, it is expensive because it contains large amounts of rare metals, and suffers unstable material supply affected by world economic conditions.
From the aspect of economic feasibility, stable material supply and efficient use of resources, heat-resistant cast steels for exhaust members desirably have necessary heat resistance with the amounts of rare metals minimized. Thus provided are inexpensive, high-performance exhaust members, which enable the application of fuel-efficiency-improving technologies to inexpensive popular cars, contributing to reducing the emission of a CO2 gas. To minimize the amounts of rare metals contained, the matrix structures of alloys are advantageously ferritic rather than austenitic. In addition, because heat-resistant, cast ferritic steels have smaller coefficients of linear expansion than those of heat-resistant, cast austenitic steels, the former have better thermal cracking resistance because of smaller thermal stress generated at the start and acceleration of engines.
Because cast exhaust members are subjected to machining such as cutting in surfaces attached to engines or peripheral parts, connecting portions such as mounting holes, portions needing high dimensional precision, etc., and then assembled in automobiles, they should have high machinability. However, heat-resistant, cast steels used for exhaust members are generally difficult-to-cut materials with poor machinability, and particularly heat-resistant, cast ferritic steels have poor machinability, because they contain much Cr for high strength. Accordingly, relatively expensive cutting tools having high hardness and strength are needed to cut exhaust members made of the heat-resistant, cast ferritic steels. Because of a short tool life, tools should be exchanged frequently, resulting in a higher machining cost. Further, because slow cutting is inevitable, cutting needs a long period of time, resulting in low machining efficiency. Thus, exhaust members made of the heat-resistant, cast ferritic steels suffer low machining productivity and poor economic feasibility.
For improved castability, JP 7-197209 A proposes a heat-resistant, cast ferritic steel having excellent castability, which has a composition comprising by weight 0.15-1.20% of C, 0.05-0.45% of C—Nb/8, 2% or less of Si, 2% or less of Mn, 16.0-25.0% of Cr, 1.0-5.0% of W and/or Mo, 0.40-6.0% of Nb, 0.1-2.0% of Ni, and 0.01-0.15% of N, the balance being Fe and inevitable impurities, and has an α′ phase (α+carbide) transformed from a γ phase (austenite phase), in addition to a usual α phase (α ferrite phase), the area ratio of the α′ phase [α′/(α+α′)] being 20-70%. Because this heat-resistant, cast ferritic steel contains C (austenitizing element) in an amount more than necessary for forming NbC, C dissolved in the matrix structure forms a γ phase when solidified. The γ phase is transformed to an α′ phase in a cooling process, thereby improving ductility and oxidation resistance. Accordingly, this heat-resistant, cast ferritic steel is suitable for exhaust members used at 900° C. or higher.
In an as-cast state, however, a γ phase is not sufficiently transformed to an α′ phase, but is transformed to a martensite phase. Because the martensite phase has high hardness, it extremely deteriorates room-temperature toughness and machinability. To secure sufficient toughness and machinability, a heat treatment for precipitating the α′ phase while disappearing the martensite phase may be necessary. However, a heat treatment generally increasing a production cost nullifies the economic advantages of the heat-resistant, cast ferritic steels with low rare metal contents.
To improve machinability, WO 2012/043860 proposes a heat-resistant, cast ferritic steel having excellent melt flowability, gas defect resistance, toughness and machinability, which has a composition comprising by weight 0.32-0.45% of C, 0.85% or less of Si, 0.15-2% of Mn, 1.5% or less of Ni, 16-23% of Cr, 3.2-4.5% of Nb, Nb/C being 9-11.5, 0.15% or less of N, (Nb/20−0.1) % to 0.2% of S, and 3.2% or less in total of W and/or Mo, the balance being Fe and inevitable impurities, and a structure in which an area ratio of eutectic (δ+NbC) phase formed from δ ferrite and Nb carbide (NbC) is 60-80%, and an area ratio of manganese chromium sulfide (MnCr)S is 0.2-1.2%.
With the amounts of C and Nb increased and their balance optimized, the heat-resistant, cast ferritic steel of WO 2012/043860 has improved melt flowability because of a lowered solidification start temperature, and drastically improved toughness because of finer primary δ crystal grains and eutectic (δ+NbC) crystal grains. Further, with a proper amount of S added, manganese chromium sulfide (MnCr)S is crystallized, resulting in a lower solidification termination temperature and an expanded solidification temperature range, and thus improved gas defect resistance. However, because the heat-resistant, cast ferritic steel of WO 2012/043860 was provided for improved melt flowability, gas defect resistance and toughness, the improvement of machinability has not been sufficiently considered. Namely, though WO 2012/043860 proposes that the amounts of machinability-deteriorating alloy elements contained are restricted by the crystallization of a γ phase transformed to martensite, increase in the amount of carbides precipitated, and increase in the amounts of alloy elements dissolved in a matrix structure, etc., thereby suppressing decrease in the machinability, it does not disclose a means for improving the machinability positively.
Because the heat-resistant, cast ferritic steels of JP 7-197209 A and WO 2012/043860 have enough room for improvement in machinability as described above, a heat-resistant, cast ferritic steel having higher machinability is desired.